Hob



Aug. Mr, i923;

N. TREOJEVma-a HOB Filed Jan. 5. 1922 Aug, M, W23. Effi-@5935i N.TREoJEx/acm HOB Filed Jan. 5, 1922 E5 Sheatazwheei 2 lm/vzw PQM/aiaTrger/45% HOB Filed Jan. 3 1922 Federated idling.L 14, 1923.

rre

l Latein HOB.

Application tiled January Be it lmow'ffihat I, Ninoue 'Innoanvicrn asubject of tl', King of Yugoslavia, residingat Detroit. in the oountyoflWayne and State of Michigan, have invented rerta'in new and usefulimprovements in Hobs, of which the following is a specification, referlence being had therein to the accompanyinlbr drawing n l The inventionrelates to the manufacture .of lspiral beve gears and has for its objectthe obtainingct a construction of hob which is capable ot' generatingsuch gears. In the present state of the art bobbing is extensively usedin the manufacture of spur gearsv where the teeth and interdental spacesare of uniform cross-section, but such methods are not applicable to the'generation of bevei gears. The ditiiculties` encountered are; First,the changing pitch and changing` tooth cross-section of 'a conventionalbevel f condition.

gear-which. cannot be generated hyan ordinary heb; second, 4with spiralbevel gears, one surface of the tooth is concave and of a curvaturewhich would preclude the use of a cylindricalhob without interference.It has been proposed to use a conicalv hob, out this would require ahelical thread of varyinglead and the mechanical difliculties in themanufacture and maintenance of such a .hob are too great to render sucha method practical.4 Furthermore, even if such hobs could be made, thegears generated thereby would he defective. VI have discovered thatspiral bevel gears of n novel type, and which are also my invention, canbe manufactured -by a `bobbing process employing a hob of motrice!principles involved in theirgeneration.y As shown in Figure l, ais apoint on a line 'or string o `which is being unwound or Dnrnorr,MICHIGAN.

a, i922. serial No. 526,512.

'from a base circle or drum c and which tracesa curve d that is aninvolute of a circle. A series of equi-spaced points a, o', etc. willgenerate similar curves, all of which are parallel to eachother. Ifthetracing points, instead of being directly in the line 6, areV in aline e offset therefrom but parallel thereto and rigid therewith, asshown at f, f 'etc. in Figure 2, such poi-nts will generate curves whichare non-parallel and outwardly divergent and may be designated asextended modified involute curves. the offsetting of the line is inwardfrom thc base circle, sli-own at g in Figure 3, and then the equi-spacedpoints h, It etc. thereon will also trace non-parallel'outwardlydivergent curves which may be termed "abridged modified involute curves.If the inward offsetting of the lineis equal to the radius of the hasecircle, as shown in Figure 4, the curves' traced by theequispaced pointsa', z", ete., are true Archimedean spirals; that is, there is e.constant ratio between the vectorial'angle andthe radius vector.

It will'be seen that the curves of the three above named series arenon-parallel and are outwardly divergent. It is known .from geometrythat involutes .only (not neces sarily of a circle) form series ofparallel curves. In that case the generating line is a common normal toevery curve of the series. However, the first named curves are notcommon involutes because the generating line at every instant rotatesabout an oiiset point and, therefore, is not a normal to any curve ofthe series. Thus, the curvesare nonparallcl and the distance between twoconsecutive curves is always less than the spacing if', f/", etc., ofthegenerating line e Figure They are also outwardly divergent because theyapproach nseries of parallel` involutes asymptotically and the normalpitch (the shortest distance between two consecutive curves)'alsoasymptotically approaches its maximum value1/'f'.

Figure 5 represents the side elevation of a tapering holo A mesh-ingwith a spiral gear (lr of the improvedconstruction, 'showing that theaxial section of the hob forms a.`

1'ack.of.constant pitch and engages the gear along a 'geod'esic linethat is a conical helix whicluin development coincides with the offsetstraight line e of Figure 2. Figure 6 is a. develo ment of a hob similarto that shown in .igure 5 take-n in the pitch cone,

all of the curves of whichnre Archimedean spirals. I'f any radial line jbe taken and normals are drawn vfrom the lseveral curves at th'e'pointsof Vintersection.with-the line j, as indicated at la, kf,etcfthesenormallines Will'all extend to arouuuon point l which pointYmaybe defined as theend ot' the polar` subnormal belonging to thevector j. ,T his is a `well known propertylo the Archimedean' spiralsbecause those spirals are the only curves forfwhich the .length of thepolar subnormal is the saine for all points of.'

the curve.l Niuncrically,.the length' of the polar subnormal equals thelead of the spi.- Aral (in development) dividedl by 211, and

is always measured perpendicular to Athe' j radius vector. Also', if thespacing of the curves in Figure 6] corresponds to the spacingof thepoints i, etc.. Figure4, the one v. 2 or 3 with'lthe respective curvestangent to eachother; This relationwill be 4true'forv and boththe curvesand normal lines will coincide. "Againeif the distance of thepolarsub-normal fromltheraditil center of Figure 6 is equal to the offsettingof the lines e or g,Figures 2 and '3, and the curves are equallyspacedalong said lines, then 'Figure 6 may be placed on either of the Figures'teeth of either the extended or abridged.

modified involute type and Without interference or 'mutilation of oneportion of the gear and the' generation of another portion. t is,however, absolutely-essential that the distance between the axis of thehob and its polar sub-normal should correspond with -the dimension ofVextension or abridgment of the gear tooth, and therefore in designingthe hob Athe dimensions are selected with this' in view. It is furthervnecessary that the lnormal radius of curvature of the helicoidal 'faceof the hob which cuts the concave side Lof. the gear tooth should at alltimes be less than lthe normal radius Aot uch concave ace. i

l thread m of the hob is a-doublle thread; but

vthis is not essential as the hohl will. operate The cutting edges are"yfornied by long-if tudinally gashin'g the thread, ab indicated at n, thenumberof these` gashes determining the number of separate cuts that aretaken' vin each revolution of the hob.;

In computation ofthe polar ,subnormal of figure maybe superimposed uponthe yother Aspiral bevel. gears .comprisin -As specifically shown` linFigure 6, the' acertain hob only the'well known mathematical rules needto be followed. 4The cone angle and the lead of thread inra hob(measured alongthe sideV and notI along the axis of4 thejhob), araknown.From that the `10 developed .lead may be obtained as'equal to the leadof the hob divided by.thesine of the cone angle. :By further dividingsaid quantity by 21:, the exact .value of the polar subnormal isobtained.

It "is desirable thatthe modificationV of the tooth curves (preferablyextended in- 1' volutes) bewxactly the same .Y as the said polarsubnormal. 'Howeveiy lfor practical purposes, thereis considerablelatitude. pera0 n iittedand satisfactory gears may be 'obtained bslightly departing from the theo# retical gures. Such a departure may'be advisable whenthe `hob is larger in, .d1 eter than it would betheoreticall required. 85 In such a case a slight decrease o -the modi.ication of 4the tooth curves serves to .compensate for the oversize hob.Y

While I prefer, on accountv of vits simu plicity in construction, to usea hob which in its development is an-'Archimedean spiral, it is possibleto' form a hob of other curves of. the modified linvolute type, Litbellhg only essential to 'lect said curve so as be equal in extensionlor abridgm'ent to the 95 curve to be generawd thereby. In Figure 7 thepitch cone development ofa hob of general .involuto 'characteria shown.'Jlhe. cutting teeth p, p', p-aro ranged on the face 'of a truncatedcone bod 100 in a series of threads m; m', etc.- which form similarextended involutes Adeveloped from the lbase circle o with themodification orv offset e. The cutting 'edges are formed by aseries ofequally spaced gashes or flutes, n",4 105 etc., which are inibstantiallyperpendicular tothe thread lines mi, 1ra/,feta and may leither straightor curved. The remaining portions of the Vthread are relievedto' formcuttinged es,'as,will be'understood.v Such 110 hobs are s arpened bygrindingthe faces' ofthe tlute`s, similarly 'to the 'recess em- 4ployedfor sharpening-ofthe 'or in ary hohe. coneangle of thehob'shouldhe such that the What I claim as'my invention is: 1. A hob or'cutter for the-generation of .115

a 'tapering helix'which'in development an in comparu Y sonwith' thedevelopment of the gear gen'-`- erated therebyhas longitudinaltooth'curvoa .r traced by polnts equispaced from lines rolll- 329 ing onthegbase lcircles respectively of the hobandgear.: Y' .y

2. A holler cutter for generating spiral bevel gears comprising 'a.tapering helix which in development is an Archlmedelln 1% spiral havina .plolar subnormal-selected to correspond wit the modificationoftlnlongitudinal4 tooth curves of the gear involutes of a circle.- A

3. A'hob or Acutter 'for thel gene-ratione# 13@ spiral bevel ofconstantV pito ltooth' curves of the gear generated thereby;

4. A hob or `cutter for the generation of spiralbevel gears comprisinga'. truncated m tapering helix which .in axial section foriiis a. rackof constat -pitch and Whichin development formsl an `Archimedean spiralhaving a polar sub-normal' selectedjwith re- Sg'ect to apredeteriiiiiiedlongitudinal tooth curve of modified involute' form in the gea-r to begenerated.

. A hob or cutter for spiralbevel gears comprising a' truncated taperinghelix longitudinally gaslied to forni cutting edges, said helix forminginthe 'axial section of the hoh rack of constant pitch and indevelopmentforming Aichimedean spiral curves having tiolar sulrnorinal selectedwith reference .to predetermined longitudinal tooth curves ofinodiiedinv'olute formin the gear to be generated.

. d. A hob ori-cutter for the generation of spiral bevelgears'lcomprising a body havin tapering helical thread lwhich; in axianon 'hormish a rackpf constantl pitch and odesic section forms a rackl.modified invol generated by a rack of helix longitudinally 'element ofconstant pitch Qga'slied and transverselycutting edges.

which in developziientf forms non-parallel iite curves, said curvesbeinconstant pitch space rom and parallel to a line rolling on -a base"circle of predetermined radius. l

7. A hoh or Cutter coniprisingfa taperingA pitch .and in develop-s n intforming a spiralof the general involute type.

A hob or cutter comprising a body having a tapering'helical thread'longitudinally gashed and transversely relieved to fornioiitting edges.said thread beingcapable/ofi;l meshing with a rack element ofconstaiitpitch along a series of. geodesiclines and eral in'volute type.'

9'. A hob or' cutter comprising .a frusto-A conical body cal thread, w siralof the general in'volute'type, said t read being capable of meshingwitlh a rack. along a series ot'V geodesic 1 lines 'and beinglongitudinally relievedto form -In testiinonl whereof I affix in Y UQDLATRB@ E VCH.

rovided with a tapering heliich,' in development, forms a gaslied toforni cutting edges, said-helix forming .in geodesic section .a rack ofconstant .forming in development a spiral of the gensignature.

